Optical recording elements for recording and storing digital information are known. One of the currently popular optical recording element is the compact disc or CD. CDs and recordable CDs (CD-Rs) have transformed the personal entertainment, personal computer and data storage industries. CDs and CD-Rs have made it possible to store enormous amounts of music or data on inexpensive, reliable, mass produced media.
Digital Versatile Disc (DVD) and recordable DVD (DVD-R) optical recording elements are being developed. For reasons stated below these elements have significantly greater storage capacity than CDs.
In CDs and DVDs digital information is stored in the form of low reflective marks on an otherwise reflective background. In this format the optical information is in the form of read only memory or ROM. Optical information is not recorded in real time, but is produced by press molding. In a typical process a substrate is stamped with a master containing the digital information in an embossed form. The stamped substrate, bearing deformations caused by the embossed master, is coated with a reflective layer and then with a protective layer. In the stamped substrate areas having the deformations the reflectivity is lower than in undeformed areas.
DVD elements have significantly more stringent requirements than CDs. The recording wavelength is 635 nm instead of 780 nm; playback wavelengths are 635 nm and 650 nm instead of 780 nm; numerical aperture of the read head is 0.6 instead of 0.5; the track pitch is 0.8 .mu.m instead of 1.6 .mu.m and the minimum mark length is 0.44 .mu.m instead of 0.83 .mu.m. These changes increase the data storage capacity significantly. In DVD the thickness of the stamped substrate is only 0.6 mm instead of 1.2 mm. However a total substrate thickness of 1.2 mm is required to make DVD elements physically stiffer and the same dimension as the popular CDs. This makes them useful in the current population of DVD modified CD players. Generally this is satisfied by forming a laminate structure that includes a substrate on opposite sides of other DVD required layers. Each substrate may be stamped with stored information or only one is so stamped.
It is sometimes desirable to produce an optical recording element that can be recorded in real time. CD-R elements having this capability are known. Such elements have dye containing optical recording layers coated onto a grooved substrate. The recording layer is coated with a reflectivity layer and then a protective layer. Exposure of the element to a laser recording beam operating in the region of 780 nm forming a relatively low reflective mark.
U.S. Pat. No. 5,500,325 to Chapman et al. discloses dye mixtures for optical recording layers. The mixtures comprise metallized azo ether dyes and a second dye. U.S. Pat. No. 5,426,015 discloses optical recording layers comprising metallized azo dianions with two cationic dye counterions.
DVD-R elements are constructed and used similarly to CD-R elements. However in DVD-R elements the shorter recording and playback wavelengths necessitate the development of new and different dye based optical recording layers.